Although much has been learned about venereal syphilis in the nearly one hundred years since the discovery of its etiologic agent, Treponema pallidum subsp. pallidum, the disease remains highly enigmatic. Our limited understanding of this chronic, systemic, sexually transmitted infection reflects the many peculiarities of the syphilis spirochete, which include the fragility and unusual molecular architecture of its outer membrane, an extremely narrow mammalian host range, and its inability to replicate continuously in artificial medium. A central theme of our research program has been and will continue to be the identification and characterization of rare outer membrane proteins. There are two principal reasons why we believe that this arduous search is about to reach a successful conclusion. First, using a lipophilic photoactivatable probe, we have identified what appears to be an authentic rare outer membrane protein, designate p30.5. Second, we now have at our disposal the T. pallidum genomic sequence, an extremely powerful too which now enables us to survey in silico the entire treponemal chromosome for additional candidate outer membrane proteins. Equally important, the T. pallidum genomic sequence has provided an invaluable platform for our investigations of physiological processes (i.e., trace metal acquisition and resistance to oxidative stress) which are fundamental to host- pathogen relations during syphilitic infection. Though clearly departures from our traditional areas of concentration, these new avenues of investigation are, nonetheless, outgrowths of our longstanding commitment to delineating in molecular terms the interface between the spirochete and its obligate human host. During the current funding interval, we have made substantial progress towards our long term objective of relating T. pallidum membrane biology to syphilis pathogenesis. In the present proposal, we hope to extend this process by integrating molecular, ultrastructural, and physiological information into a more complete picture of how the parasite sustains itself within the hostile host milieu. To achieve this end, we will molecularly characterize T. pallidum rare outer membrane proteins (Specific Aim One); define the roles of the Tro and TP0034 transporters in trace metal acquisition by T. pallidum (Specific Aim Two); and characterize key components of T. pallidum's defense against oxidative stress (Specific Aim Three).